1. Technical Field of the Invention
The present invention relates generally to an improved structure of an electronic device equipped with a resinous casing, a terminal disposed within the casing, an electronic element, and a wire joining between the terminal and the electronic element, which is designed to decrease fatigue of the wire.
2. Background Art
FIGS. 8(a) and 8(b) illustrate a semiconductor device as an example of the above type of electronic devices. The semiconductor device is designed as a pressure sensor and has disposed thereon a semiconductor diaphragm sensor element 30 which is sensitive to atmospheric pressure acting thereon to provide an electrical signal indicative thereof.
The pressure sensor consists of a molded resinous casing 10, a plurality of metallic terminals 20, and wires 50. The terminals 20 extend through the casing 10 to have ends 21 and 22 exposed outside the casing 10. The sensor element 30 is mounted on the surface of the casing 10 and joined electrically to the terminals 20 through the wires 50 for output of the electrical signal indicative of the atmospheric pressure acting on the sensor element 30.
The above structure of the pressure sensor, however, encounters the drawback as discussed below.
When the pressure sensor is subjected to thermal cycling, i.e., a rise or a drop in ambient temperature, it will cause the resin forming the casing 10 to be expanded or contracted.
The resin has a relatively great coefficient of thermal expansion, thus resulting in a great degree of expansion or contraction of the casing 10. This causes the terminals 20, as shown in FIG. 8(b), to be moved laterally in a direction Y parallel to the surface on which the sensor element 30 is mounted, thus resulting in fatigue of the wires 50, which may lead to breakage of the wires 50.
It is therefore a principal object of the invention to avoid the disadvantages of the prior art.
It is another object of the invention to provide an improved structure of an electronic device designed to minimize fatigue of wires joining between an electronic element and a terminal arising from a temperature change of a casing.
According to one aspect of the invention, there is provided an electronic device which may serve as a semiconductor pressure sensor. The electronic device comprises: (a) a casing having a body made of resin; (b) a metallic terminal having a length, the metallic terminal extending through the body of the casing to have ends thereof extending outside the body of the casing; (c) an electronic element mounted on a surface of the body of the casing; (d) a wire establishing an electrical connection between the electronic element and the end of the terminal; and (e) a buffer disposed adjacent the terminal within the body of the casing. The buffer is made of a material having a coefficient of thermal expansion smaller than that of the resin forming the body of the casing. Specifically, the buffer serves to suppress expansion or contraction of a portion of the body of the casing near the terminal which arises from a change in temperature of the casing, thereby decreasing undesirable movement of the terminal leading to fatigue of the wire. This minimizes a drop in mechanical strength of the wire caused by thermal cycling to which the casing is subjected.
In the preferred mode of the invention, the electronic device may also include a plurality of metallic terminals each having a length and ends exposed outside the body of the casing and wires connected between the ends of the metallic terminals and the electronic element. The buffer occupies between two of the terminals and/or between each of the terminals and an outer peripheral surface of the body of the casing. If a distance between the middle between the two of the terminals and each of the terminals and a distance between each of the terminals and the middle between each of the terminals and the outer peripheral surface of the casing are defined as L, the buffer occupies over 0.9L from each of the middles to the terminals. This positional relation between the buffer and the terminals maximizes the above activities of the buffer.
The buffer may be made of one of a metal and a ceramic material.